1. Field of the Invention
The invention relates to surface acoustic wave (SAW) devices. More particularly, the invention relates to SAW devices, having diamond films, used in high frequency operations.
2. Description of the Related Art
Surface acoustic wave (SAW) devices are useful, e.g., for filtering unwanted frequency signals, for providing signal delay lines, and for generating high frequency oscillations in telecommunication and other apparatus and systems. SAW devices typically include a smooth piezoelectric substrate such as quartz, lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3), and an inter-digital-transducer (IDT) having a set of alternating or interpenetrating finger-shaped electrodes formed on the piezoelectric substrate surface. The electrodes are driven by a radio frequency (RF) source. A portion of the electrode pattern includes the input transducer, which converts an incoming electrical signal appearing across the input terminals to an alternating electric field and consequently to an acoustic wave traveling along the surface of the piezoelectric substrate. The surface acoustic wave is sampled continuously by another IDT acting as an output transducer.
SAW devices having frequency-selective elements are used as acoustic filters, with the sound radiation of certain frequencies being transmitted through the pass-band structure while other frequencies are attenuated. SAW filters typically are of a resonator type or a transversal type, depending on the device structure and the mode of operation.
Descriptions of SAW devices are included in articles such as C. Hartman and S. Wilkus, "Proc. 37.sup.th Annual Frequency Control Symposium", 1993, p. 354; R. Weigel et al., "Proceedings 1996 IEEE Microwave Theory and Technology Conference", June 1996, Paper No. WE1A-4, p. 413; Y. Taguchi et al., "Proceedings 1996 IEEE Microwave Theory and Technology Conference", June 1996, Paper No. WE1A-5, p. 417. These articles hereby are incorporated by reference herein. Also, a discussion of conventional SAW devices is included in the book M. Feldmann and J. Henaff, Surface Acoustic Waves for Signal Processing, Artech House, Boston, 1996, chapter 4.
A filter using a SAW device has a theoretical maximum filtering response at the signal frequency f=.nu..sub.p /.lambda., where .nu..sub.p is the acoustic phase velocity of the sound-transmitting medium and .lambda. is the geometric spacing in the IDT fingers. Diamond has a relatively high acoustic velocity, i.e., greater than 11,000 m/sec, and hence allows SAW devices incorporating diamond to be operated at higher frequencies. The use of diamond films as substrates for SAW devices is disclosed generally in articles such as Nakahata et al., "High Frequency Surface Acoustic Wave Filter Using ZnO/Diamond/Si Structure", Japan. Journal of Appl. Phys. Vol. 33, 324 (1994); and Shikata et al., "SAW Filters Based on Diamond", Proc. Applied Diamond Conf. 1995, Applications of Diamond Films and Related Materials, p. 29. Because diamond is not piezoelectric, an additional layer of piezoelectric material typically is used in SAW devices having diamond film substrates.
Although single crystal diamond films or high-quality heteroepitaxially-grown diamond films often are desirable, producing such films over a large area with convenient, relatively low-cost processing is not commercially available at present. Therefore, conventional SAW devices comprising diamond use relatively thin chemical vapor deposition (CVD) diamond films deposited directly on a silicon (Si) substrate. Such diamond films typically are thin, e.g., no more than approximately 50 microns (.mu.m), and typically exhibit a very fine grain size, e.g., approximately 1-5 .mu.m. See, for example, the article Graebner et al., "Large Anisotropic Thermal Conductivity in Synthetic Diamond Films", Nature, vol. 359, 401 (1992).
However, the deposition of a thin, fine-grained diamond film directly on a silicon substrate, as is done with conventional diamond SAW devices, induces additional complications of having to polish the rough diamond film surface, which often contaminates and/or stresses the delicate silicon device circuitry and associated components. Furthermore, the grain boundary area in diamond films is a disturbed surface and therefore is a source of acoustic wave attenuation, which causes a loss of efficiency in wave transmission.
Accordingly, it is desirable to have available a SAW device having large-grained diamond films incorporated therein for greater operating efficiency. Also, it is desirable to have available a convenient method for incorporating large-grained diamond films into SAW devices. Desirably, the diamond films coincide with the integrated circuit (IC) circuitry and the IDT transducer electrode patterns without suffering from conventional grain boundary induced losses and processing complications.